1. Field of the Invention
This invention relates to a composition for cleaning the surfaces of aluminum and aluminum alloys, which is particularly useful for the removal of lubricating oil and smut from the surfaces of aluminum cans.
2. Description of Related Art
Products with aluminum surfaces, e.g., beverage containers made of aluminum or aluminum alloy, are ordinarily manufactured by a molding operation known as drawing and ironing (below, called "DI process"). During this molding operation, lubricating oil is applied to the metal surface, and smut adheres to the resulting container, especially to its inner walls. The surface of this kind of container is generally protected afterwards by, e.g., a conversion coating treatment or painting, and prior to this treatment it is necessary to remove the above-mentioned lubricating oil or smut from the metal surface. Ordinarily, an acid cleaner is used in this surface cleaning.
Conventionally, as the acid cleaner, a chromic acid type cleaner has been used in order to prevent corrosion of the treatment apparatus, but the use of these cleaners has been eliminated because of the toxicity of the chromium ion. As substitutes, hydrofluoric acid cleaning agents have been proposed. For example, according to U.S. Pat. No. 3,728,188, a cleaning agent has been proposed which consists of an acidic aqueous solution containing 0.5-2.0 g/l fluoride ion, 5-21 g/l ferric ion, and 0.05-3.0 g/l thiourea, the pH of which is regulated to 0.1-1.8 with a strong mineral acid such as sulfuric acid, etc. With this cleaner, satisfactory surface cleaning is accomplished due to the fact that the large quanity of fluoride ions causes a rapid rate of etching the aluminum, while on the other hand, this etching is inhibited by the ferric ions.
However, in the case of fluoride ions special care must be employed with respect to preventing pollution of the working environment and waste liquid treatment, due to their toxicity. This is also true, not only for the case in which hydrofluoric acid is used, but also for systems in which other fluorides are present which introduce fluoride ions into the cleaner.
Cleaners with small quantities of fluoride ions, which are a problem in this respect, have also been known conventionally. For example, according to a British Pat. No. 1,454,974, a cleaner has been proposed which consists of an aqueous acidic solution containing 0.005-0.1 g/l fluoride ions and 1-10 g/l sulfuric acid, and which has a pH of 1.0-1.8. Although the fluoride ion content is reduced in this way, its toxicity can by no means be neglected. Moreover, in this cleaner, the cleaning power is somewhat reduced along with the reduction in fluoride ion content.